Ion exchange is used in the glass industry to change the properties of a bulk glass substrate to create, for example, optical waveguides or to increase the strength of the glass. For example, Corning's GORILLA® glass comprises an aluminosilicate bulk glass where potassium ions are exchanged for sodium ions in a surface layer using a diffusion process. The diffusion process is carried out using, for example, an aluminosilicate glass substrate immersed in a potassium salt bath that is maintained at an elevated temperature.
Because the potassium ions are larger than the sodium ions, a compressive stress arises in the ion-exchange region of the aluminosilicate substrate. This compressive stress serves to strengthen the glass, thereby making GORILLA® glass and related ion-exchanged glasses useful for a host of applications requiring a strong glass with scratch-resistant surface. Example applications include flat-panel cover glass for laptop computers, televisions and hand-held displays.
The strength of an ion-exchanged glass is largely defined by its stress profile, which is a measure of the induced stress as a function of distance into the glass from the glass surface. Unfortunately, present-day non-destructive and efficient stress measurement techniques only provide a surface stress measurement or a crude linear approximation for the stress profile, and such measurements have limited utility. When manufacturing an ion-exchanged glass, a non-destructive measurement of the full stress profile, or a more accurate representation of it than the linear approximation, can provide a more complete understanding of how process conditions lead to particular stress profile, and how the different stress profiles correlate with glass strength and frangibility. Such measurement capability could improve and accelerate the development of new and improved ion-exchanged glasses and ion exchange processing.